Institute of Electrical and Electronics Engineers (IEEE) 802.11 is a set of physical and media access control (MAC) specifications for implementing wireless local area network (WLAN) communications. These specifications provide the basis for wireless network products using the Wi-Fi brand managed and defined by the Wi-Fi Alliance. The specifications define the use of the 2.400-2.500 GHz as well as the 4.915-5.825 GHz bands. These spectrum bands are commonly referred to as the 2.4 GHz and 5 GHz bands. Each spectrum is subdivided into channels with a center frequency and bandwidth. The 2.4 GHz band is divided into 14 channels spaced 5 MHz apart, though some countries regulate the availability of these channels. The 5 GHz band is more heavily regulated than the 2.4 GHz band and the spacing of channels varies across the spectrum with a minimum of a 5 MHs spacing dependent on the regulations of the respective country or territory.
Communication on any given channel of either the 2.4 GHz or the 5 GHz band between network elements of the WLAN utilizes the clear channel assessment (CCA) protocol. CCA is defined in the IEEE 802.11 standard as part of the Physical Medium Dependent (PMD) and Physical Layer Convergence Protocol (PLCP) layer. Clear Channel Assessment is composed of two related functions, carrier sense (CS) and energy detection (ED).
Carrier sense (CS) refers to the ability of the receiver to detect and decode an incoming Wi-Fi signal preamble. In addition, CCA must be reported as BUSY when another Wi-Fi signal preamble is detected, and must be held as BUSY for the length of the received frame as indicated in the frame's PLCP Length field. Typically, any incoming Wi-Fi frame whose PLCP header can be decoded will cause CCA to report the medium as busy for the time required for the frame transmission to complete.
The PLCP header Length field indicates either the number of microseconds required for transmission of the full frame MAC protocol data unit (MPDU) payload, or the number of octets carried in the frame MPDU payload which is then used in combination with the Rate field (which identifies the modulation used for the payload) to determine the time required for MPDU transmission. In any case, the length or rate+length fields of the MPDU give the receiver the information required to de-modulate the frame and determine how long the wireless medium will be busy.
Energy detection (ED) refers to the ability of the receiver to detect the energy level present on a given channel where a discernable Wi-Fi preamble cannot be decoded. ED is based on the noise floor, ambient energy, interference sources, and unidentifiable Wi-Fi transmissions that may have been corrupted but can no longer be decoded. ED cannot predict the exact length of time the wireless medium will be busy, instead ED must sample the wireless medium in every slot time to determine if the energy still exists. ED utilizes a threshold level above which the detected energy level must exceed before the wireless medium is classified as busy or idle. This minimum threshold level can be referred to as the ED threshold level or CCA sensitivity level. The CCA sensitivity level is usually much lower for valid Wi-Fi signals that can be decoded using CS than it is for other signals where a discernable preamble cannot be decoded.